In the process of making acrylic acid the most widely used processes are those which oxidize propylene or acrolein to acrylic acid. Many different combinations of metal oxides have been used as catalysts. Most of these contain molybdenum oxide as the principal component. Some of the catalyst are effective in oxidizing the propylene directly to acrylic acid although this is frequently a function of the conditions under which the catalyst is used; others oxidize acrolein to the acid. In either case, any acrolein not converted, or acrolein made in the process of oxidizing propylene to acrylic acid, may be recycled to the feed stream and subsequently oxidized to form the acrylic acid.
In addition to molybdenum oxide, the catalysts of the prior art contain many other metals (usually as their oxides) which promote the catalytic effect of the molybdenum. The transition metals of Group VIII of the periodic chart, including iron, cobalt and nickel, have been employed in many such catalysts. Others selected from various groups of metals of the periodic chart have been employed. Thus, for example, titanium, vanadium, chromium, tungsten and manganese in groups IVB, VB, VIB, and VIIB are known to be useful. From group IIA such metals as magnesium, calcium, strontium and barium; from groups IB and IIB copper, silver, zinc and cadmium; also from groups IA and VA sodium, potassium, antimony and bismuth; all have been taught as catalytic promotor components. Phosphorous has been employed, usually added as phosphoric acid or as metal phosphates, as an essential part of many of the prior art catalysts. Some of the early catalysts employed molybdenum oxide in combination with 1 to 2 other metals, e.g. U.S. Pat. No. 2,881,212 which employed several metals as phosphomolybdates. In more recent years, catalysts containing 3-6 or as many as 8-10 different metals have been disclosed.
The art known to the invention as being closest to that of the present invention is found in U.S. Pat. Nos. 3,775,474; 3,833,649; 3,886,092 which teach the use of Mo, V, Cr, Cu and W in various combinations. None of these teach the use of tantalum or niobium as taught by the present invention, nor does the instant catalyst composition contain the tungsten taught by the above three U.S. patents. Another U.S. Pat. No. 3,865,873 employs tantalum together with molybdenum and phosphorous, but contains none of the other components of the catalyst of this invention.
The efficiency of the catalysts made from any particular combination of metals apparently is affected by the manner in which they are made, and whether or not it is supported or in pelleted form. The porosity and the surface area of either the pellet or the support are important to the performance of the catalyst and will determine to some extent the amount of catalytic material employed on the support. It is extremely important in the preparation of the catalyst to obtain uniform distribution of the various oxides contained therein, otherwise the effect of the combination may be lost. The molybdenum and the various promoter metals are added as their soluble salts, usually in acid solution. They are sometimes mixed together in the same solution, but because of possible problems with premature recipitation which would cause non-uniformity in the finished catalyst the metals are most frequently made up in separate solutions which are in turn added together under the proper controlled conditions. Generally, the final pH of the solutions is slightly on the acid side, at about pH 6-6.5. The solvent is then evaporated from the solution of the catalyst components in the presence of a support in order to burden it, when a support is used. If not supported, the components are dried and pelleted. Most of the techniques associated with the manufacture of the catalyst are well known to the prior art. The particular techniques used to make the catalyst of the present invention will be enumerated and exemplified herein.
The parameters of feed composition, flow rate, temperature and pressure are well known to the prior art. Thus the aldehyde is usually present in the feed stream from about 1-10 volume %, the remainder being molecular oxygen (0.8 to 21 vol. %) and inert gas. Steam is frequently used as the inert gas although since air is usually employed as the source of oxygen, nitrogen is also present as an inert gas component. Temperatures employed are within the range of 200.degree.-400.degree.C and pressures of from about 1-10 atm. are commonly used. Contact times are usually on the order of 0.4 to about 15 seconds depending upon the temperature employed and the efficiency of the particular catalyst.